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UC3717AQ Scheda tecnica(PDF) 4 Page - Texas Instruments |
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UC3717AQ Scheda tecnica(HTML) 4 Page - Texas Instruments |
4 / 12 page The UC3717A’s drive circuit shown in the block diagram includes the following components. (1) H-bridge output stage (2) Phase polarity logic (3) Voltage divider coupled with current sensing compa- rators (4) Two-bit D/A current level select (5) Monostable generating fixed off-time (6) Thermal protection OUTPUT STAGE The UC3717A’s output stage consists of four Darlington power transistors and associated recirculating power di- odes in a full H-bridge configuration as shown in Figure 7. Also presented, is the new added feature of inte- grated bootstrap pull up, which improves device per- formance during switched mode operation. While in switched mode, with a low level phase polarity input, Q2 is on and Q3 is being switched. At the moment Q3 turns off, winding current begins to decay through the commu- tating diode pulling the collector of Q3 above the supply voltage. Meanwhile, Q6 turns on pulling the base of Q2 higher than its previous value. The net effect lowers the saturation voltage of source transistor Q2 during recircu- lation, thus improving efficiency by reducing power dissi- pation. UC3717A Figure 6. UC3717A Test Circuit Figure 7. Simplified Schematic of Output Stage Note: Dashed lines indicate current decay paths. FUNCTIONAL DESCRIPTION 4 |
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